1. Field of the Invention
This invention relates to a tapered roller bearing, particularly to a tapered roller bearing suitable for incorporation into the gear device of the transmission of an automobile.
2. Brief Description of the Prior Art
Transmissions for automobiles are classified broadly into the manual type and the automatic type. Further, they can also be classified according to the driving system of the vehicle: a trans-axle for front wheel drive (FWD), a transmission for rearwheel drive (RWD), and a transfer for four-wheel drive (4WD). They are used to speed-change the drive power from the engine and to transmit it to the drive shaft or the like.
FIG. 7 shows an example of the arrangement of the transmission of an automobile. This transmission is of the synchronous type, in which the left side is the engine side and the right side is the drive wheel side in the same figure. A tapered roller bearing 43 is interposed between a main shaft 41 and a main drive gear 42. In this example, the inner periphery of the main drive gear 42 is directly formed with an outer ring raceway surface for the tapered roller bearing 43. The main drive gear 42 is supported by a tapered roller bearing 44 for rotation relative to a casing 45. A clutch gear 46 is connected by engagement to the main drive gear 42, and a synchro-mechanism 47 is disposed adjacent the clutch gear 46.
The synchro-mechanism 47 comprises a sleeve 48 adapted to be moved axially (in a left-right direction in the same figure) by the action of a selector (not shown), a synchronizer key 49 axially slidably installed in the inner periphery of the sleeve 48, a hub 50 connected by engagement to the outer periphery of the main shaft 41, a synchronizer ring 51 slidably mounted on the outer periphery (the cone section) of the clutch gear 46, and a hold-down pin 52 and a spring 53 which elastically press the synchronizer key 49 against the inner periphery of the sleeve 48.
In the state shown in the same figure, the sleeve 48 and synchronizer key 49 are held in the neutral position by the hold-down pin 52. At this time, the main drive gear 42 runs idle relative to the main shaft 41. On the other hand, when the selector is actuated to cause the sleeve to move axially, for example, to the left, from the state shown in the same figure, the synchronizer key 49 moves axially to the left following the sleeve 48, so as to press the synchronizer ring 51 against the inclined surface of the cone section of the clutch gear 46. This decreases the rotative speed of the clutch gear 46 and reversely, increases the rotative speed of the synchro-mechanism 47. And, at about the time when the rotative speeds of the two have synchronized, the sleeve 48 further moves axially to the left, meshing with the clutch gear 46, and the main shaft 41 and the main drive gear 42 are connected to each other through the synchro-mechanism 47. This allows the main shaft 41 and the main drive gear 42 to synchronously rotate.
In this connection, automobile transmissions have recently tended to use low-viscosity oil so as to attain AT, CVT, low fuel consumption, etc., for the mission. In an environment where low viscosity oil is used, surface-start flaking, which results in a very short life, sometimes occurs in the inner ring raceway surface due to poor lubrication when such adverse conditions as (1) high oil temperature, (2) little oil quantity, and (3) loss of pre-load, simultaneously happen.
As for measures against this short life due to surface-start flaking, a direct and effective solution is to reduce the maximum surface pressure. To reduce the maximum surface pressure, it is necessary to change the bearing size or to increase the number of rollers of the bearing if the bearing size is not to be changed. To increase the number of rollers without decreasing the roller diameter, it is necessary to narrow the distance between pockets in the cage. To this end, however, the pitch circle of the cage has to be increased to draw the cage as close as possible to the outer ring.
As an example in which the cage is drawn to the side until it touches the inner diameter surface of the outer ring, there is a tapered roller bearing shown in FIG. 8 (refer to Patent Document 1: Japanese Patent Laid-Open No. 2003-28165). In this tapered roller bearing 61, the outer peripheral surfaces of the small diameter and large diameter annular sections 62a and 62b, respectively, of the cage 62 are placed in slide contact with the inner diameter surface of the outer ring 63 so as to guide the cage 62, and the outer diameter surface of the pole section 62c of the cage 62 is formed with a recess 64 for suppressing drag torque, thereby maintaining the non-contact state between the outer diameter surface of the pole section 62c and the raceway surface 63a of the outer ring 63. The cage 62 has the small diameter annular section 62a, the large diameter annular section 62b, and a plurality of pole sections 62c which axially connect the small diameter annular section 62a and the large diameter annular section 62b and which is formed with a recess 64 in the outer peripheral surface thereof. And disposed between pole sections 62c are a plurality of pockets for rollably receiving tapered rollers 65. The small diameter annular section 62a is provided with a flange section 62d integrally extending to the inner diameter side. The tapered roller bearing of FIG. 8 is an example intended to improve the strength of the cage 62, wherein the cage 62 is drawn to the side until it touches the inner diameter surface of the outer ring 63 in order to increase the peripheral width of the pole section 62c. 